100 DEVELOPMENT AND HEREDITY. 



frequently repeated become fixed in their relation 

 to each other, and though the stimuli may vary 

 slightly from what they originally were, yet the 

 original series of actions will still result. The varia- 

 tion of the stimuli, however, if constantly repeated, 

 finally makes itself felt in a variation of the actions 

 of the series. 



Professor Jos. Jastrow has recognised this law 

 of nervous action of which we have treated in this 

 chapter, and he has excellently described it as 

 what he calls the law of habit. "This law de- 

 clares that every reaction of an organism to a condi- 

 tion in its environment renders the repetition of 

 that reaction quicker, easier, more certain, more 

 uniform ; and the existence of habits implies an 

 environment sufficiently constant to repeatedly pre- 

 sent to the organism the same or closely similar 

 conditions. Mere existence in a world so full of 

 regularities, of rhythm and law, of recurrence of 

 the same needs, results in the performance of defi- 

 nite actions in definite ways ; and it equally results 

 that the earliest experiences will produce the 

 strongest impressions, and will gradually render more 

 difficult the learning of other modes of reaction, 

 even though these others, owing to a change of 

 conditions, would be more useful."^ 



' " Problems of Comparative Psychology," Professor Jastrow, Popu- 

 lar Science Monthly, November, 1892. 



